Drilling super-hot horizons of the Larderello geothermal field: insights from noble gases

Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse (IGG), 56124 Pisa, Italy

^* Corresponding author: f.gherardi@igg.cnr.it

Abstract

In the frame of EU-funded DESCRAMBLE project, the Venelle 2 well was deepened down to 2900 m b.g.l. to test novel drilling techniques, and to improve the knowledge of physical-chemical conditions of super-hot horizons in the Larderello geothermal field, Central Italy. The major seismic reflectors present in the area were the target of the drilling, to verify their potential as geothermal reservoirs of supercritical fluids. No significant fluid entries were observed during the operations. Noble gases were analysed in the emerging drilling fluids to determine whether deep-seated fluids with a different signature than those currently discharged by productive wells entered the borehole. Despite the predominant atmospheric signature of the noble gas suite, ³He/⁴He values up to 1.59 R_a were measured, and the contribution of a deep component with ~4 R_a was hypothesised by extrapolation of binary mixing trends.